Method for manufacturing single sheet-type green sheet, method for manufacturing silicon nitride sintered body, single sheet-type green sheet, and silicon nitride sintered body

ABSTRACT

A method for manufacturing a single sheet-type green sheet includes a transporting step of transporting a strip-shaped green sheet that contains ceramic along a longitudinal direction thereof, and an irradiation step of irradiating the transported strip-shaped green sheet with a laser beam to cut the strip-shaped green sheet, thereby obtaining a single sheet-type green sheet.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a singlesheet-type green sheet, a method for manufacturing a silicon nitridesintered body, a single sheet-type green sheet, and a silicon nitridesintered body.

BACKGROUND ART

It has been known that a ceramic board is used as a base material of acircuit board for a power module. Such a ceramic board is manufacturedsuch that a strip-shaped green sheet made of ceramic is cut to obtainsingle sheet-type green sheets, the obtained single sheet-type greensheets are laminated to obtain a laminate, and the laminate obtained bythe single sheet-type green sheets being laminated is sintered.

Here, in Patent Document 1, a method for manufacturing a ceramiclaminate including a cutting step of cutting a ceramic strip-shapedgreen sheet to obtain a plurality of single sheet-type green sheets, alaminating step of laminating the plurality of single sheet-type greensheets obtained by the cutting, and a sintering step of sintering theplurality of single sheet-type green sheets obtained by the laminatingis disclosed. In the above-mentioned cutting step, the plurality ofsingle sheet-type green sheets are obtained by cutting the strip-shapedgreen sheet using a plate-shaped cutting blade.

RELATED DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Publication No.2017-065059

SUMMARY OF THE INVENTION Technical Problem

As described above, in the manufacturing method disclosed in PatentDocument 1, the strip-shaped green sheet is cut using the cutting bladein the cutting step. Thus, this manufacturing method has the followingproblems. That is, chips may be generated from the cut surfaces of thesingle sheet-type green sheets by a cutting operation using the cuttingblade, and the cutting blade maybe worn by the repeated cuttingoperation, so that metal powder may be generated. In addition, in a casewhere the laminating step is performed with the generated chips andmetal powder adhering to the surfaces of the single sheet-type greensheets, unevenness defects occur in the single sheet-type green sheets.Furthermore, during the cutting of the strip-shaped green sheet usingthe cutting blade, the cutting blade moves while pressurizing thestrip-shaped green sheet with a blade edge. Therefore, the cut surfacesof the single sheet-type green sheets, which are formed by the cuttingwith the cutting blade, are formed while the strip-shaped green sheet ispulled by the blade edge. As a result, cracks may be generated near thecut surfaces of the single sheet-type green sheets.

As described above, the generation of unevenness defects and cracksresults in a problem of a decrease in the yield of the ceramic board.

An object of the present invention is to provide a method formanufacturing a single sheet-type green sheet in which chips from a cutsurface and cracks near the cut surface are hardly generated, and metalpowder due to a cutting blade is not generated in a case where thestrip-shaped green sheet is cut to obtain the single sheet-type greensheets.

Solution to Problem

A method for manufacturing a single sheet-type green sheet of a firstaspect of the present invention includes a transporting step oftransporting a strip-shaped green sheet that contains ceramic along alongitudinal direction thereof, and an irradiation step of irradiatingthe transported strip-shaped green sheet with a laser beam to cut thestrip-shaped green sheet to obtain a single sheet-type green sheet.

According to the manufacturing method of a second aspect of the presentinvention, in the method for manufacturing a single sheet-type greensheet of the first aspect, the laser beam with which the strip-shapedgreen sheet is irradiated in the irradiation step is emitted from anirradiation portion that emits a carbon dioxide laser beam.

According to the manufacturing method of a third aspect of the presentinvention, in the method for manufacturing a single sheet-type greensheet of the first or second aspect, a step of performing doctor blademolding or extrusion molding on a slurry containing ceramic powder tohave a strip shape to obtain the strip-shaped green sheet, is the stepbeing performed before the transporting step, is further included.

According to the manufacturing method of a fourth aspect of the presentinvention, in the method for manufacturing a single sheet-type greensheet of the third aspect, the ceramic powder includes silicon nitridepowder or aluminum nitride powder.

According to a method for manufacturing a silicon nitride sintered bodyof the present invention, the single sheet-type green sheet that ismanufactured by the method for manufacturing a single sheet-type greensheet according to any one aspect of the first to fourth aspects isheated and sintered to obtain a silicon nitride sintered body.

The single sheet-type green sheet according to the first aspect of thepresent invention has a laser cut surface on at least one side surface.

According to the single sheet-type green sheet of the second aspect ofthe present invention, in the single sheet-type green sheet of the firstaspect, the laser cut surface has a surface roughness Ra of equal to orgreater than 0.5 μm and equal to or smaller than 2.0 μm, and the lasercut surface has a surface roughness Rz of equal to or greater than 5.0μm and equal to or smaller than 12.0 μm.

A silicon nitride sintered body of the present invention is formed in asheet shape and includes an end surface, in which the end surface has asurface roughness Ra of equal to or greater than 0.5 μm and equal to orsmaller than 2.0 μm, and the end surface has a surface roughness Rz ofequal to or greater than 5.0 μm and equal to or smaller than 12.0 μm.

Advantageous Effects of Invention

According to the method for manufacturing a single sheet-type greensheet of the present invention, chips from the cut surface and cracksnear the cut surface are hardly generated, and metal powder due to thecutting blade is not generated in a case where the strip-shaped greensheet is cut to obtain the single sheet-type green sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the presentinvention will be more apparent from the following description ofcertain preferred embodiments taken in conjunction with the accompanyingdrawings.

FIG. 1 is a flow chart illustrating a manufacturing process of a ceramicboard of the present embodiment.

FIG. 2A is a diagram for explaining a molding step of the presentembodiment, and is a schematic view for explaining a state in which astrip-shaped green sheet is produced from a slurry using a doctor blademolding device.

FIG. 2B is a diagram for explaining a cutting step of the presentembodiment, and is a schematic view (side view) for explaining a statein which the strip-shaped green sheet is cut using a cutting device toproduce a single sheet-type green sheet.

FIG. 2C is a schematic view of FIG. 2B when seen from the front.

FIG. 2D is a schematic view of a laminate of the present embodiment.

FIG. 3 is an SEM image of a cut surface of the single sheet-type greensheet of the present embodiment.

FIG. 4 is a table summarizing conditions and observation results of afirst test.

FIG. 5A is an enlarged photograph of the vicinity of a cut surface of asingle sheet-type green sheet of a first comparative example, in whichthe photograph is seen from an upper surface side in a thicknessdirection.

FIG. 5B is an SEM image of the cut surface of the single sheet-typegreen sheet of the first comparative example.

FIG. 6 is an enlarged photograph of the vicinity of a cut surface of asingle sheet-type green sheet of a second comparative example, in whichthe photograph is seen from an upper surface side in a thicknessdirection.

FIG. 7 is an enlarged photograph of the vicinity of a cut surface of asingle sheet-type green sheet of a third comparative example, in whichthe photograph is seen from an upper surface side in a thicknessdirection.

FIG. 8 is an enlarged photograph of the vicinity of a cut surface of asingle sheet-type green sheet of a fourth comparative example, in whichthe photograph is seen from an upper surface side in a thicknessdirection.

FIG. 9 is an enlarged photograph of the vicinity of the cut surface ofthe single sheet-type green sheet of the present embodiment, in whichthe photograph is seen from an upper surface side in a thicknessdirection.

FIG. 10 is a table summarizing conditions and measurement results of athird test.

DESCRIPTION OF EMBODIMENTS Summary

First, a method for manufacturing a silicon nitride sintered body 40(see FIG. 2D) of the present embodiment will be described with referenceto the drawings. Next, Example of the present embodiment will bedescribed with reference to the drawings. In all the drawings referencedin the following description, the same components are denoted by thesame reference numerals, and the description thereof will not berepeated as appropriate.

Here, the silicon nitride sintered body 40 is, for example, a ceramicboard for a power module mounted on an electric vehicle, a railroadvehicle, or other industrial equipment. The silicon nitride sinteredbody 40 is obtained such that single sheet-type green sheets 30 (seeFIGS. 2B and 2C) described later are sintered in a laminated state, forexample. Each of the single sheet-type green sheets 30 is obtained suchthat a strip-shaped green sheet 20 is cut (see FIGS. 2A and 2B). Thatis, a relationship between the silicon nitride sintered body 40 and thesingle sheet-type green sheet 30 is a relationship between a finishedproduct and an intermediate product (a product manufactured in a stepbefore the finished product is obtained) or a relationship between afirst intermediate product and a second intermediate product (a productmanufactured in a step before the first intermediate product isobtained). Therefore, the single sheet-type green sheet 30 of thepresent embodiment is manufactured through steps at an intermediatestage of the method for manufacturing the silicon nitride sintered body40 of the present embodiment, which will be described later. Therefore,the method for manufacturing the single sheet-type green sheet 30 of thepresent embodiment will be described in the description of the methodfor manufacturing the silicon nitride sintered body 40 of the presentembodiment.

Method for Manufacturing Silicon Nitride Sintered Body of PresentEmbodiment

Hereinafter, the method for manufacturing the silicon nitride sinteredbody 40 of the present embodiment will be described with reference toFIG. 1, FIG. 2A, FIG. 2B, and FIG. 2C.

As illustrated in a flow chart of FIG. 1, the method for manufacturingthe silicon nitride sintered body 40 of the present embodiment includesa slurry producing step, a molding step, a cutting step, a depositingstep, a degreasing step, and a sintering step, and these steps arecarried out in the described order.

In the flow chart of FIG. 1, the method for manufacturing the siliconnitride sintered body 40 is denoted by reference numeral S10, and theslurry producing step, the molding step, the cutting step, thedepositing step, the degreasing step, and the sintering step are denotedby reference numerals S11, S12, S13, S14, S15, and S16, respectively.

Slurry Producing Step

First, a slurry producing step S11 will be described. This step is astep of mixing raw material powder with an organic solvent describedlater to produce a slurry 10. The slurry 10 produced in this step (seeFIG. 2A) is molded into the strip-shaped green sheet 20 in the next step(molding step).

The raw material powder of the slurry 10 is a powder containing a maincomponent and a sintering aid, which will be described later. The maincomponent is, for example, silicon nitride (Si₃N₄) of 80% by weight to98.3% by mass, and the sintering aid is, for example, at least one rareearth element of 1% by weight to 10% by mass (expressed in terms ofoxide) and magnesium (Mg) of 0.7% by weight to 10% by mass (expressed interms of oxide). A ratio at which silicon nitride powder is of anα-phase is preferably 20% to 100% in consideration of the density,bending strength, and thermal conductivity of the silicon nitridesintered body 40. Here, to give further details about the meaning of“to” used in the present specification, for example, “20% to 100%” means“equal to or more than 20% and equal to or less than 100%”. In addition,“to” used in this specification means equal to or more than thedescription before “to” and equal to or less than the description after“to”.

The reason why a ratio of silicon nitride (Si₃N₄) in the raw materialpowder is 80% by weight to 98.3% by mass as an example is that thebending strength and thermal conductivity of the obtained siliconnitride sintered body 40 are not too low, a denseness of the siliconnitride sintered body 40 due to lack of the sintering aid is ensured,and the like.

The reason why the ratio of at least one rare earth element in the rawmaterial powder is 1% by weight to 10% by mass (expressed in terms ofoxide) is that in a case where a ratio thereof is less than the ratio of1% by weight to 10% by mass, the bond between the silicon nitrideparticles is weakened and cracks easily extend at grain boundaries, andin a case where a ratio is more than the ratio of 1% by weight to 10% bymass, the bending strength decreases and a ratio of grain boundaryphases increases and the thermal conductivity decreases. The reason whythe ratio of magnesium (Mg) in the raw material powder is 0.7% by weightto 10% by mass (expressed in terms of oxide) is that in a case where aratio thereof is less than the ratio of 0.7% by weight to 10% by mass, aliquid phase produced at a low temperature is insufficient, and in acase where a ratio thereof is more than the ratio of 0.7% by weight to10% by mass, the volatilization amount of Mg increases and holes arelikely to be formed in the silicon nitride sintered body 40.

Here, a content of Mg is preferably 0.7% by weight to 7% by mass(expressed in terms of oxide), and more preferably 1% by weight to 5% bymass. In addition, a content of at least one rare earth element ispreferably 2% by weight to 10% by mass (expressed in terms of oxide).Therefore, a content of Si₃N₄ is preferably 83% by weight to 97.3% bymass, and more preferably 90% by weight to 97% by mass. As the rareearth element, Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu,and the like can be used, but Y is preferably used from the viewpoint ofan increase in the density of the silicon nitride sintered body 40. Itis preferable to use each of Mg and at least one rare earth element inthe form of oxide powder.

Therefore, the sintering aid is preferably a combination of MgO powderand Y₂O₃ powder.

Hereinafter, for the simplification of the description, the raw materialpowder of silicon nitride is referred to as Si₃N₄ powder (also known asan example of silicon nitride powder or ceramic powder), the rawmaterial powder of Mg is referred to as MgO powder, and the raw materialpowder of the rare earth element is referred to as Y₂O₃ powder. However,as described above, the raw material powder of silicon nitride and theraw material powder of the sintering aid may not be Si₃N₄ powder, andMgO powder and Y₂O₃ powder, respectively.

The Si₃N₄ powder, MgO powder, and Y₂O₃ powder blended as described aboveare mixed with a plasticizer, an organic binder, and an organic solventto produce the slurry 10. Therefore, the slurry 10 produced in this stepcontains ceramic powder. Here, examples of the plasticizer include aphthalic acid-based plasticizer such as di-n-butylphthalate, a dibasicacid-based plasticizer such as di2-ethylhexyl sebacate, and the like.Examples of the organic binder include ethyl cellulose, polyvinylbutyral, an acrylic-based binder, and the like. Examples of the organicsolvent include ethyl alcohol, toluene, acetone, MEK, and the like. Asolid content concentration of the slurry 10 produced in this step ispreferably 30% by weight to 70% by mass from the viewpoint of ease ofmolding in the next step (molding step).

The above is the description of the slurry producing step.

Molding Step

Next, a molding step S12 will be described. This step is a step ofproducing the strip-shaped green sheet 20 from the slurry 10 asillustrated in FIG. 2A.

This step is carried out using a doctor blade molding device 100illustrated in FIG. 2A. Here, the doctor blade molding device 100 isprovided with a belt transporting mechanism 110, a molding unit 120, anda heating unit 130. The belt transporting mechanism 110 includes aroller 112A on an upstream, a roller 112B on a downstream, and a belt114, and movement of the belt 114 from the roller 112 on the upstream tothe roller 112 on the downstream (along an X direction) is made by thedrive of the roller 112 on the downstream. The molding unit 120 isdisposed on an upper side of the belt 114 (on a Z direction side of thebelt 114) and faces the belt 114. The molding unit 120 is provided withan accommodating portion 122 accommodating the slurry 10 and a doctorblade 124.

In addition, as illustrated in FIG. 2A, the molding unit 120 forms thesheet-shaped slurry 10 having a film thickness, which is obtained suchthat the slurry 10 is brought out from the accommodating portion 122 dueto its own weight and an adhesive force between the slurry 10 and themoving belt 114, and is regulated and defined by the doctor blade 124.The heating unit 130 blows warm air WC onto the slurry 10 having thedefined film thickness on the belt 114 to form the slurry 10 into asheet (the organic solvent is vaporized). As a result, in the moldingstep, the strip-shaped green sheet 20 having a width defined from theslurry 10 (a Y direction in the drawing corresponds to a widthdirection) is produced. That is, in the molding step, the slurry 10 isformed into a strip shape by the doctor blade molding to obtain thestrip-shaped green sheet 20 containing Si₃N₄ (ceramic) as an example.

As an example, this step is carried out after defoaming the slurry 10that is produced in the slurry producing step S11 and thickening theslurry 10. In addition, the film thickness of the strip-shaped greensheet 20 produced in this step is set in consideration of a filmthickness of the silicon nitride sintered body 40 to be finallyproduced. Along with this, regulation conditions (a distance from thebelt 114, and the like) of the doctor blade 124 for regulating theslurry 10 to have the defined film thickness are also set inconsideration of the film thickness of the silicon nitride sintered body40 to be finally produced. Here, the film thickness of the strip-shapedgreen sheet 20 to be set is set to, for example, 0.25 mm to 1 mmdepending on the film thickness of the silicon nitride sintered body 40to be finally produced, but 0.25 mm to 0.9 mm is preferable, and 0.25 to0.8 mm is more preferable.

The above is the description of the molding step.

Cutting Step

Next, a cutting step S13 will be described. This step is a step ofcutting the strip-shaped green sheet 20 to produce the single sheet-typegreen sheet 30 as illustrated in FIG. 2B.

This step is performed using a cutting device 200 illustrated in FIG.2B. Here, the cutting device 200 is provided with a sheet transportingmechanism 210 and a cutting portion 220.

The sheet transporting mechanism 210 includes a supporting portion 212,a first transporting portion 214, and a second transporting portion 216.The supporting portion 212 rotatably supports the roller 112B (see FIGS.2A and 2B) with the strip-shaped green sheet 20 produced in the moldingstep being wound around an outer peripheral surface thereof. The firsttransporting portion 214 adjusts the posture of the strip-shaped greensheet 20 transported from the supporting portion 212 and transports thestrip-shaped green sheet 20 to the cutting portion 220 along the Xdirection (along the longitudinal direction of the strip-shaped greensheet 20). The second transporting portion 216 transports the singlesheet-type green sheet 30 produced by the strip-shaped green sheet 20being cut at the cutting portion 220 to further downstream (to the Xdirection).

The cutting portion 220 includes a housing 222, an irradiation portion224, and a moving mechanism 226. The irradiation portion 224 emits acarbon dioxide laser beam LB (an example of a laser beam) as an example.The moving mechanism 226 causes the irradiation portion 224 to scan thestrip-shaped green sheet 20 from one end to the other end thereof in thelateral direction (the Y direction in the drawing). The irradiationportion 224 and the moving mechanism 226 are mounted in the housing 222.

Then, in the cutting device 200 of the present embodiment, the sheettransporting mechanism 210 allows the strip-shaped green sheet 20 to betransported by the length of the single sheet-type green sheet 30 and tostop the transportation of the strip-shaped green sheet 20, and thestrip-shaped green sheet 20 is cut by the cutting portion 220. In thiscase, the cutting portion 220 causes the irradiation portion 224 to emitthe carbon dioxide laser beam LB while causing the moving mechanism 226to move the irradiation portion 224 from one end side to the other endside of the strip-shaped green sheet 20 in the lateral direction alongthe Y direction. In addition, the irradiation portion 224 that performsthe scanning through the moving mechanism 226 intermittently emits thecarbon dioxide laser beam LB. Here, “intermittently” means to repeatirradiation for a certain time period and non-irradiation for a certaintime period. Therefore, the moving mechanism 226 causes the irradiationportion 224 to move and stop repeatedly so as to cause the irradiationportion 224 to perform the scanning.

As described above, in this step, the strip-shaped green sheet 20 isirradiated with the carbon dioxide laser beam LB to cut the strip-shapedgreen sheet 20, so that the single sheet-type green sheet 30 isobtained. In addition, this step also includes a step of transportingthe strip-shaped green sheet 20 by the sheet transporting mechanism 210(transporting step), and a step of cutting the strip-shaped green sheet20 by the cutting portion 220 to obtain the single sheet-type greensheet 30 (irradiation step).

In the explanation of this step, an example of a laser beam LB is thecarbon dioxide laser beam LB. However, a laser beam having a wavelengthdifferent from the wavelength of the carbon dioxide laser beam LB may beused as long as the light beam emitted from the irradiation portion 224is a laser beam. For example, an infrared laser beam LB (IR laser beamLB), an ultraviolet laser beam LB (UV laser beam LB), and the like maybe used. However, as in this step, the laser beam LB emitted from theirradiation portion 224 is preferably a carbon dioxide laser beam LB.The reason for this will be described in the description of Exampledescribed later. FIG. 3 is an SEM image of a cut surface 32 (an exampleof a laser cut surface) of the single sheet-type green sheet 30 producedby the cutting step of the present embodiment, and details of this SEMimage will also be described later in the description of Exampledescribed later.

The above is the description of the cutting step.

Depositing Step

Next, a depositing step S14 will be described. This step is a step oflaminating a plurality of the strip-shaped green sheet 20 in the filmthickness direction as illustrated in FIG. 2D. This step is a stepperformed for efficiently sintering the single sheet-type green sheet 30in the subsequent step (sintering step S16).

In this step, as illustrated in FIG. 2D, the plurality of the singlesheet-type green sheets 30 are deposited by interposing a non-reactivepowder layer (not illustrated) therebetween, which will be describedlater. Here, in a case where the number of the single sheet-type greensheets 30 to be laminated is small, the number of sheets that can beprocessed in a sintering furnace (not illustrated) at one time in thesubsequent sintering step S16 is small (the production efficiencydecreases). On the other hand, in a case where the number of the singlesheet-type green sheets 30 to be laminated is large, binders containedin the single sheet-type green sheets 30 are unlikely to be decomposedin the next step (degreasing step S15). According to the above reasons,the number of the single sheet-type green sheets 30 to be laminated inthis step is 8 to 100, and preferably 30 to 70.

The non-reactive powder layer of the present embodiment is, for example,a boron nitride powder layer (BN powder layer) having a film thicknessof about 1 μm to 20 μm. The BN powder layer has a function of easilyseparating the silicon nitride sintered body 40 after the subsequentstep (sintering step S16). The BN powder layer is applied as a slurry ofBN powder on one surface of each single sheet-type green sheet 30 by,for example, spraying, brush coating, roll coater, screen printing, orthe like. The BN powder has a purity of 85% or more, and preferably hasan average particle diameter of 1 μm to 20 μm.

The above is the description of the depositing step.

Degreasing Step

Next, a degreasing step S15 will be described. This step is a step ofdegreasing a binder and a plasticizer contained in the single sheet-typegreen sheet 30 before the next step (sintering step S16).

In this step, as an example, a plurality of the single sheet-type greensheets 30 (see FIG. 2D) laminated in the depositing step S14 are held ina temperature environment of 450° C. to 750° C. for 0.5 hours to 20hours. As a result, the binders and the plasticizers contained in theplurality of the single sheet-type green sheets 30 are degreased.

The above is the description of the degreasing step.

Sintering Step

Next, a sintering step S16 will be described. This step is a step ofsintering the plurality of the single sheet-type green sheets 30laminated in the depositing step S14 (hereinafter, referred to as theplurality of single sheet-type green sheets 30 in FIG. 2D) using asintering device (not illustrated).

The sintering device is provided with a sintering furnace and a controldevice that controls a temperature of the sintering furnace. Thesintering furnace includes a heater and a thermometer. Then, in thisstep, the plurality of single sheet-type green sheets 30 illustrated inFIG. 2D are accommodated in the sintering furnace, and the heater as anexample is controlled by the control device according to a temperaturecontrol program described later.

Here, the temperature control program is a program that is stored in astorage device (for example, ROM or the like) included in the controldevice, and that controls the temperature of the heater while referringto a temperature profile based on temperature information of thethermometer included in the sintering furnace (for example, PID controlor the like). Specifically, the temperature control program is a programsetting the temperature profile in the sintering furnace as a profilethat is formed with a temperature increasing region with a graduallyheating region, a temperature maintaining region, and a cooling region,which proceed in the described order. Hereinafter, the technicalsignificance of the gradually heating region, the temperaturemaintaining region, and the cooling region will be described.

Temperature Maintaining Region

The temperature maintaining region is a temperature region whererearrangement of silicon nitride particles, production of β-type siliconnitride crystals, and grain growth of silicon nitride crystals areenhanced from the liquid phase produced in the gradually heating regionto further densify the sintered body.

The temperature in the temperature maintaining region is preferably setto a temperature within the range of 1600° C. to 2000° C. and themaintaining time is preferably 1 hour to 30 hours in consideration of asize and an aspect ratio (a ratio of the major axis to the minor axis)of β-type silicon nitride particles, formation of holes due tovolatilization of the sintering aid, and the like. In a case where thetemperature in the temperature maintaining region is lower than 1600°C., the silicon nitride sintered body 40 is difficult to be densified.On the other hand, in a case where the temperature higher than 2000° C.,the sintering aid volatilizes and the silicon nitride decomposesviolently, which makes the silicon nitride sintered body 40 difficult tobe densified. In a case where the temperature in the temperaturemaintaining region is the temperature within the range of 1600° C. to2000° C., a heating temperature in the temperature maintaining regionmay be set to change with time (for example, the temperature isgradually increased).

Here, the temperature in the temperature maintaining region is morepreferably a temperature in the range of 1750° C. to 1950° C., and evenmore preferably a temperature in the range of 1790° C. to 1890° C.Furthermore, the temperature in the temperature maintaining region ispreferably equal to or 50° C. higher than the upper limit of atemperature in the gradually heating region, and more preferably equalto or 100° C. to 300° C. higher than the upper limit thereof. Themaintaining time of the temperature maintaining region is morepreferably 2 hours to 20 hours, and even more preferably 3 hours to 10hours.

The above is the description of the sintering step. In addition, theabove is the description of the method for manufacturing the siliconnitride sintered body 40 of the present embodiment.

Example

Next, Example of the present embodiment (tests for deriving thepreferable form of the present embodiment) will be described withreference to the drawings. Here, the tests for which the preferable formof the present embodiment has been derived are a first test, a secondtest, and a third test described below. The effects of the presentembodiment described above will be described inconsideration of a resultof each test described later.

First Test

Hereinafter, a first test will be described.

Method of First Test

In this test, a test of observing a photograph of a cut section of thesingle sheet-type green sheet 30 produced in the cutting step S13 of thepresent embodiment and photographs of cut sections of single sheet-typegreen sheets produced in cutting steps of comparative examples (a firstto fourth comparative examples) described below was performed.Specifically, cut sections (cut surfaces and peripheral portionsthereof) of a sample of the single sheet-type green sheet 30 of thepresent embodiment and samples of the single sheet-type green sheets ofthe first to fourth comparative examples each were photographed from thelower surface side, and observed. Then, the enlarged photograph of eachsample was observed to confirm the presence or absence of cutting chipsand burrs on the cut surface. As a result, the sample with at least oneof cutting chips or burrs was regarded as unacceptable, and the samplewithout both was regarded as acceptable.

Here, the single sheet-type green sheet (see FIGS. 5A and 5B) of thefirst comparative example was produced such that the strip-shaped greensheet 20 was cut by an extrusion blade (not illustrated). The singlesheet-type green sheet (see FIG. 6) of the second comparative examplewas produced such that the strip-shaped green sheet 20 was cut by aThomson blade (not illustrated). The single sheet-type green sheet (seeFIG. 7) of the third comparative example was produced such that thestrip-shaped green sheet 20 was cut by shirring (not illustrated). Thesingle sheet-type green sheet (see FIG. 8) of the fourth comparativeexample was produced such that the strip-shaped green sheet 20 was cutby an ultrasonic cutter (not illustrated). The single sheet-type greensheet 30 of the present embodiment was produced by using an infraredlaser (IR laser) provided with the irradiation portion 224 (see FIGS. 2Band 2C).

Result and Discussion of First Test

FIG. 4 is a table summarizing conditions and observation results of eachsample of this test.

The first to fourth comparative examples were all unacceptable. Here, inthe first comparative example (extrusion blade), cutting chips wereobserved (see FIGS. 4, 5A, and 5B). In the second comparative example(Thomson blade), burrs were observed (see FIGS. 4 and 6). In the thirdcomparative example (shirring), cutting chips were observed (see FIGS. 4and 7). In the fourth comparative example (ultrasonic cutter), cuttingchips were observed (see FIGS. 4 and 8).

The present embodiment (laser) was acceptable (see FIGS. 3, 4, and 9).

It is presumed that fracture surfaces were generated during the cuttingsince all of the first to fourth comparative examples were produced by acontact-type cutting unit. On the other hand, it is presumed that afracture surface was not generated (or hardly generated) during thecutting as in the case of the first to fourth comparative examples sincethe present embodiment is produced by a non-contact-type cutting unit.

As described above, according to the method for manufacturing the singlesheet-type green sheet 30 of the present embodiment, chips from the cutsurface are not generated (or hardly generated) in a case where thestrip-shaped green sheet 20 is cut to obtain the single sheet-type greensheet 30. In addition, according to the method for manufacturing thesingle sheet-type green sheet 30 of the present embodiment, the fracturesurface is not generated (or hardly generated) and cracks near the cutsurface also are not generated (or hardly generated) during the cutting.In the method for manufacturing the single sheet-type green sheet 30 ofthe present embodiment, metal powder from the cutting blade is notgenerated since the cutting blade is not used during the cutting. Alongwith these, according to the method for manufacturing the siliconnitride sintered body 40 of the present embodiment, the yield is higherthan that in the case where the contact-type cutting unit is used in thecutting step.

The above is the description of the first test.

Second Test

Next, the second test will be described.

Method of Second Test

In this test, the single sheet-type green sheet 30 was produced using acarbon dioxide laser, an infrared laser (IR laser), and an ultravioletlaser (IJV laser) as the irradiation portion 224 (see FIGS. 2B and 2C),the cut surface 32 (see FIG. 2B and FIG. 3) was observed, and timeperiods required for cutting operations (processing time) were comparedto each other.

Here, center wavelengths of a laser beam LB of the carbon dioxide laserwere 9360 nm and 10600 nm, a center wavelength of a laser beam LB of theinfrared laser was 1064 nm, and a center wavelength of the laser beam LBof the ultraviolet laser was 355 nm.

Result and Discussion of Second Test

Cutting chips and burrs were not observed on the cut surface 32 (lasercut surfaces) in any cases (a result is the same result as in the firsttest illustrated in Table of FIG. 4). In addition, the processing timeat the same output was shorter in the order of carbon dioxide laser,infrared laser, and ultraviolet laser (a graph of the test result or thelike is not described).

Therefore, in the cutting step S13 of the present embodiment, chips andburrs from the cut surface 32 are not generated (or hardly generated)even using any lasers as the irradiation portion 224. However, from theviewpoint of shortening the processing time, it can be said that thecarbon dioxide laser, the infrared laser, and the ultraviolet laser arepreferably used in this order.

Regarding the infrared laser and the ultraviolet laser, the former has ashorter processing speed than that of the latter, and the reason ispresumed to be as follows. That is, it is presumed that thermalprocessing with respect to the cutting of the strip-shaped green sheet20 is preferentially performed before optical processing, and the laserbeam LB from the infrared laser is more easily converted due to heatthan the laser beam LB from the ultraviolet laser. Here, a bandgap ofthe strip-shaped green sheet 20 is about 5.0 eV, whereas a bandgap ofthe ultraviolet laser is 3.5 eV corresponding to the center wavelengthof 355 nm. Therefore, it is presumed that the laser beam LB from theultraviolet laser is difficult to sufficiently photoexcite thestrip-shaped green sheet 20.

The above is the description of the second test.

Third Test

Next, the third test will be described.

Method of Third Test

In this test, samples each of which was obtained such that three singlesheet-type green sheets 30 that have been cut in the cutting step S13 ofthe present embodiment were sintered in the sintering step S16, andsamples each of which was obtained such that three single sheet-typegreen sheets that have been cut using press processing (extrusion blade)in the cutting step were sintered in the sintering step S16 wereproduced. Subsequently, surface roughnesses Ra, Ry, and Rz of thesesamples were measured based on JIS B 0601-1994.

Here, each of the carbon dioxide laser, the infrared laser, and theultraviolet laser was used for cutting each sample of the presentembodiment in the cutting step S13.

Result and Discussion of Third Test

Table in FIG. 10 summarizes conditions and measurement results of thethird test. This test is not a test for identifying the quality of eachsample from the measurement results of the surface roughnesses Ra, Ry,and Rz of each sample. However, from the result of the first test, itcan be said that the silicon nitride sintered body 40 manufactured inthe steps including the cutting step S13 of the present embodimentsatisfies the measurement result in each sample using the carbon dioxidelaser, the infrared laser, and the ultraviolet laser in Table of FIG.10.

Therefore, it can be said that in the silicon nitride sintered body 40of the present embodiment, the surface roughness Ra of the end surface42 (see FIG. 2D, in other words, the laser cut surface after sinteringor the sintered surface of the laser cut surface) is equal to or greaterthan 0.5 μm and equal to or smaller than 2.0 μm, and the surfaceroughness Rz of the end surface 42 is preferably equal to or greaterthan 5.0 μm and equal to or smaller than 12.0 μm. Along with this, itcan be said that in the single sheet-type green sheet 30 of the presentembodiment, the surface roughness Ra of the cut surface 32 (see FIGS. 2Band 3) is equal to or greater than 0.5 μm and equal to or smaller than2.0 μm, and the surface roughness Rz of the cut surface 32 is preferablyequal to or greater than 5.0 μm and equal to or smaller than 12.0 μm.

The above is the description of Example of the present embodiment.

As described above, the present embodiment has been described as anexample of the present invention, but the present invention is notlimited to the present embodiment. The technical scope of the presentinvention also includes, for example, the following forms(modifications).

For example, in the description of the present embodiment, an example ofthe ceramic powder has been described as silicon nitride. However, anexample of the ceramic powder may be another ceramic powder. Forexample, aluminum nitride may be used.

In the description of the molding step S12 of the present embodiment, itis assumed that the molding step S12 is performed by doctor blademolding. However, as long as the slurry 10 can be molded into thestrip-shaped green sheet 20, another method may be adopted as themolding method. For example, extrusion molding may be used.

In the description of the cutting step S13 of the present embodiment,the strip-shaped green sheet 20 is cut while the irradiation portion 224moves from one end side to the other end side of the strip-shaped greensheet 20 in the lateral direction. However, as long as the singlesheet-type green sheet 30 can be obtained by cutting the strip-shapedgreen sheet 20 as a result, a cut portion of the strip-shaped greensheet 20 may not be a linear portion crossing over from one end side tothe other end side of the strip-shaped green sheet 20 in the lateraldirection as in the case of the present embodiment. For example, thestrip-shaped green sheet 20 may be cut such that a hole that has a shapeof the single sheet-type green sheet 30 is made in the strip-shapedgreen sheet 20 to separate (or hollow out) the single sheet-type greensheet 30 from the strip-shaped green sheet 20. That is, the singlesheet-type green sheet 30 obtained by cutting the strip-shaped greensheet 20 may have at least a part of all end surfaces thereof as a cutsurface.

Priority is claimed on Japanese Patent Application No. 2019-013761,filed Jan. 30, 2019, the disclosure of which is incorporated herein itsentirety by reference.

1. A method for manufacturing a single sheet-type green sheet comprisingan irradiation step of irradiating a strip-shaped green sheet thatcontains ceramic with a laser beam to cut the strip-shaped green sheetto obtain a single sheet-type green sheet.
 2. The method formanufacturing a single sheet-type green sheet according to claim 1,wherein the laser beam with which the strip-shaped green sheet isirradiated in the irradiation step is emitted from an irradiationportion that emits a carbon dioxide laser beam.
 3. The method formanufacturing a single sheet-type green sheet according to claim 1,further comprising a transporting step of transporting the strip-shapedgreen sheet to the irradiation step along a longitudinal direction ofthe strip-shaped green sheet.
 4. The method for manufacturing a singlesheet-type green sheet according to claim 3, further comprising a stepof performing doctor blade molding or extrusion molding on a slurrycontaining ceramic powder to have a strip shape to obtain thestrip-shaped green sheet, the step being performed before thetransporting step.
 5. The method for manufacturing a single sheet-typegreen sheet according to claim 4, wherein the ceramic powder includessilicon nitride powder or aluminum nitride powder.
 6. A method formanufacturing a silicon nitride sintered body comprising heating andsintering the single sheet-type green sheet that is manufactured by themethod for manufacturing a single sheet-type green sheet according toclaim 1 to obtain a silicon nitride sintered body.
 7. A singlesheet-type green sheet having a laser cut surface on at least one sidesurface.
 8. The single sheet-type green sheet according to claim 7,wherein the laser cut surface has a surface roughness Ra of equal to orgreater than 0.5 μm and equal to or smaller than 2.0 μm, and the lasercut surface has a surface roughness Rz of equal to or greater than 5.0μm and equal to or smaller than 12.0 μm.
 9. A silicon nitride sinteredbody formed in a sheet shape comprising an end surface, wherein the endsurface has a surface roughness Ra of equal to or greater than 0.5 μmand equal to or smaller than 2.0 μm, and the end surface has a surfaceroughness Rz of equal to or greater than 5.0 82 m and equal to orsmaller than 12.0 μm.